The first important factor is volume of your nutrient reservoir. That is deciding how many gallons of nutrient solution you need for each plant in the system, total in the reservoir. Larger plants use more nutrients and water than smaller plants do. So use your plants as a guide as to how much nutrient solution is needed to properly supply them with what they need. For instance for large plants like tomatoes you should estimate at least 2.5 gallons of nutrient solution per plant in the system. For medium size plants like peppers or basil estimate at least 1.5 gallons of nutrient solution per plant in the system. Then for smaller plants like strawberries or lettuce estimate at least 1/2 gallon of nutrient solution per plant in the system. Then multiply that by how many plants you plan to grow in order to give you a minimum reservoir size.
This is important because larger volumes of nutrient solution will have smaller fluctuations in their nutrient concentrations as the plants use them up. Basically this helps keep the nutrient solution evenly balanced as the plants deplete the nutrients from it. Also in warm/hot weather the plants transpire (use) more water than they do in cool weather. As the plants use up more of the water from the reservoir than usual, this concentrates the nutrients (EC levels) that are left in the solution. With smaller water volumes there is a much better chance of nutrient levels soaring to toxic levels/concentrations. Even when regularly topping off with fresh water to the original volume in smaller reservoirs, there is a lot of fluctuation of the nutrient concentrations that wont do your plants any good.
There are a lot of variables that can affect the amount of nutrients the manufacture recommends to use (dosage). Of coarse some trial and error here can be helpful to you in the long run, but you should start with what the manufacturer recommends. After all, they know their nutrients best. Some of the things that can affect the dosage that the nutrient manufacturer recommends to use are plants size (seedlings, small plants, heavy growth etc.), the plants growth phase (like vegetative growth or flowering/fruiting growth), type of growing medium (clay pellets, Coco cor, Perlite, Vermiculite etc.).
Even the type of system like recirculating or non-recirculating systems can make a difference. Most manufactures assume that your plants are going to be grown in favorable conditions, so they don’t always post directions for weather extremes for those people that grow outdoors. But like I mentioned earlier, the plants will take up more water in hot environments, and going a little under the recommendations (dosage) for hot conditions is generally recommended.
When mixing your nutrients and/or additives, add all the water (at least 3/4 of it) to the reservoir first. Two and three part nutrients are separated for a reason. If you pre mix them in concentrated form or even small amounts of water first, some of the elements in the solution that were separated now have a chance to bond, and a white precipitate (calcium sulphate) may even be seen forming in the solution. By thoroughly mixing each concentrated part of the nutrient (and/or additives) one at a time into the larger volume of water will keep them diluted enough so they wont have a chance to bond together. If they do, they will become un-usable to the plants.
Keep your nutrients mixed up. For most systems this wont be much of a problem, using a air bubbler in the reservoir will easily keep the water moving. Also most systems that recirculate the nutrient solution should also keep the water mixed up enough. This keeps the mineral elements from settling and concentrating at the bottom. This also prevents areas/pockets of high or low pH levels, where the mineral elements then become unusable outside of the plants pH range.
Check your pH often. Plants are only able to uptake the nutrients in the solution if the pH is in the right range. After mixing your nutrients immediately check the pH, and adjust it if necessary. Then recheck it until it’s right. After that checking it daily would be recommended. Letting the pH go out of range will deprive your plants of nutrients, and allowing pH to rise above 6.5 can often cause the white precipitate to be seen in nutrient reservoirs also.
It’s not just having good nutrients that maters, it’s also how you use them that makes the most difference in their performance. By taking the time to regularly fallow a few basic guidelines, as well as checking the manufactures instructions, then fallowing their instructions correctly. You will be able to get the most out of any nutrients you choose to use. Growing good hydroponic plants requires a little more work than just pouring some water on a mound of dirt, but well worth the effort.